In a navigation system for automobiles which is gaining popularity recently, for example, the position of an automobile on which the system is mounted is determined by utilizing an information from GPS (Global Positioning System) satellites. Such a positioning technology is disclosed in Japanese Laid-Open Patent Application No. 137,009/86, for example.
Satellites transmit information which indicate their predetermined orbits and which indicate an exact timing when the information transmission from the satellites takes place. Such information can be received on earth to calculate the positions of the satellites on a basis of such information. In addition, a distance between a particular satellite and a point of reception can be calculated from a propagation time delay of radio wave. Accordingly, when information from four satellites is available simultaneously, for example, if equations which are derived from respective satellite information can be put into simultaneous equations, the latter may be solved to detect three-dimensional position (longitude, latitude and elevation) of the point of reception as well as an error of a clock provided on the part of a receiver in an accurate manner. If the elevation of the point of reception is known already, the longitude, the latitude and the error of the clock can be detected on the basis of the information available from three satellites. In addition, such positioning is also possible if the number of available satellites is equal to or less than two, by detecting an acceleration of a vehicle onboard and using the vehicle acceleration in an auxiliary manner.
However, when a GPS receiver is used in a hostile environment for the radio wave such as in a street, an anomalous solution (in respect to the position, the speed and azimuth) may be produced. One of reasons herefor is considered as a result of a positioning calculation which is made by using extraordinary radio waves as caused by multi-path (for example, reflected waves from a high-rise building). In Japanese Laid-Open Patent Applications No. 137,009/86 and No. 167,886/86, there is proposed a positioning system which is provided with a gyro, a speedometer and an altimeter or an azimuth instrument for supplementing wanting information when the number of available satellites is reduced, but this system can not compensate for an error in the positioning calculation which results when receiving extraordinary radio waves as caused by multi-path.
In Japanese Laid-Open Patent Application No. 43,446/95 (a corresponding U.S. patent application Ser. No. 08/614,941, filed Aug. 2, 1994, and presently pending), the applicant has proposed a positioning system which verifies the correctness of information received by a GPS receiver. In this arrangement, a distance between the receiver and each of three or more GPS satellites in a group of GPS satellites from which a reception is possible is determined on the basis of positional information of GPS satellites and a receiver to derive a first distance. A time required for the radio wave to propagate from a GPS satellite to the receiver is calculated, and is multiplied by the speed of propagation (which is the velocity of light) to provide a second distance. When there is a large difference between the first and the second distance, it is assumed that there is at least one GPS satellite in the group, the information transmitted from which is in error. Accordingly, one of GPS satellites in the group is replaced by another in the set to repeat a similar calculation and an error check. In this manner, a particular GPS satellite which transmitted erroneous information is identified, and is registered as unusable while the remaining GPS satellites which have been determined as transmitting correct information are registered as usable GPS satellites. A positioning calculation is then made by utilizing only usable GPS satellites. In this manner, GPS satellite or satellites which have undergone on offset from the orbit to cause a deviation between its actual position and information transmitted thereby is removed from indices which are used in the positioning calculation, thus reducing an error which is caused in the positioning calculation.
However, this represents a verification of the correctness of information transmitted by GPS satellites, and its believed to be unsatisfactory to avoid an anomalous solution when receiving extraordinary radio waves as caused by multi-path (for example, reflected waves from high-rise building) because it is impossible to identify which information of received data from GPS satellites is erroneous as a result of radio receiving environment on earth, in particular a local environment. In addition, the calculation of the first and the second distance is repeated by changing one of the GPS satellites in the group by another which is out of the group, which takes a relatively long time interval.